Preheat the oven to 425 degrees F (220 degrees C). Coat an 8x8-inch baking pan with Nonstick Cooking Spray (to taste).
Place the Butter (2 tablespoon) in a baking pan, then place pan in the oven to melt the butter. This should only take a few minutes - keep an eye on it so it doesn’t burn.
In a food processor, mixer, or blender, combine the Egg (4), Cream Cheese (4 ounce), Coconut Flour (1/4 cup), Granulated Erythritol (1/4 cup), Baking Powder (1 1/2 teaspoon), Vanilla Extract (1 teaspoon), and zest of the Lemon (2) until you have a uniform batter.
Pour the batter into the baking pan on top of the melted butter. It will swirl together and bring some of the butter to the top, which is totally fine.
Drop the Fresh Blueberries (1/4 cup) on top, evenly spacing across the surface.
Bake for 15-18 minutes, or until a toothpick inserted into the center comes out clean. The cake might seem a little soft when it comes out of the oven, but will firm up as it cools off thanks to the erythritol.
Serve warm. Top with a little bit of extra butter if desired.